The present invention relates to a combustion state detecting system for use in an internal combustion engine.
As one of conventional systems for detecting the combustion state in an internal combustion engine, there is known a knock detecting apparatus which is arranged to detect the knocking state of an internal combustion engine. In this knock detecting apparatus, the knock detecting operation is effected such that the signal generated from a knock sensor provided in the internal combustion engine is converted by an A/D converter into digital data which is in turn processed through a microcomputer so as to detect the occurrence of knock in the internal combustion engine. In this case, in order to propagate the output signal of the knock sensor to the microcomputer with a high accuracy, a signal whose level is arranged to be as high as possible within a range that does not cause the overflow is required to be inputted to the A/D converter. However, the level of the output signal of the knock sensor is greatly affected by the rotational speed of the internal combustion engine where the knock sensor is provided. For instance, in the case that the rotational speed of the internal combustion engine is in a low state, the output signal level of the knock sensor is extremely low, and on the other hand, when the rotational speed of the engine increases, the output signal level becomes extremely high. Thus, the knock detecting apparatus is required to be equipped with an amplifier having a plurality of amplification factors to amplify or attenuate the output signal thereof. Conventionally, the switching control of the amplification factor in the amplifier is performed on the basis of the maximum value (knock strength value) of the output signal of the knock sensor as disclosed in the Japanese Patent provisional Publication No. 64-45967.
Furthermore, as a different system for detecting a combustion state in an internal combustion engine, there is known an apparatus in which a cylinder internal-pressure sensor is used to detect the internal pressure in a cylinder of the internal combustion engine and the detected cylinder internal-pressure sensor signal is supplied through an A/D converter to a microcomputer to detect occurrence of misfire (for example, Japanese Patent Provisional Publication No. 60-45750).
However, in such a knock detecting apparatus, irrespective of the same operating condition that the rotational speed of the engine is constant, the output signal of the knock sensor greatly varies at every ignition cycle so that the knock strength value irregularly varies. Particularly, the knock sensor output signal at the time of the occurrence of knock becomes increased in level up to about 16 times as compared with the output signal at the time of no occurrence of knock, and in addition the knock sensor output signal irregularly and greatly varies even at the same knock decision interval. Accordingly, there is a problem which arises such a conventional amplification-factor switching control apparatus in that the amplification factor is required to be set to a relatively low value for prevention of the overflow so that difficulty is encounter to effectively detect the knock. Similarly, there is a problem which arises with the apparatus for detecting misfire on the basis of the internal pressure in the cylinder of the internal combustion engine in that the amplification factor for the cylinder internal-pressure sensor signal is set to a relatively low value for prevention of the overflow in inputting the cylinder internal-pressure sensor signal to the A/D converter, thereby making it difficult to effectively detect the misfire due to deterioration of accuracy.